Recently, we described the isolation and properties of a T3 phage coded protein which specifically inhibits host RNA polymerase and not phage specific T3 RNA polymerase. Further evidence that this protein functions in vivo has been obtained by examining the kinetics of beta- galactosidase synthesis after T3 phage infection. Apart from the production of the inhibitory protein, T3 phage also elicits the modification of host RNA polymerase in infected cells. Preliminary studies indicate that two T3 phage mutants which map to the left of Gene 1 failed to produce the modification of beta subunit. During the course of the purification of RNA polymerase we have isolated a novel poly(A) polymerase that is different in its properties from polynucleotide phosphorylase and primer dependent poly(A) polymerase of E. coli.